<html lang="en">
<head>
<meta charset="UTF-8">
 <meta http-equiv="X-UA-Compatible" content="IE=edge">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>Document</title>
</head>
<body>
 <input type="text" id="n1">
 <select name="" id="opt">
 <option value="+">+</option>
 <option value="-">-</option>
 <option value="*">*</option>
 <option value="/">/</option>
 </select>
 <input type="text" id="n2">
 <span onclick="count()">=</span>
 <span id="res"></span>
</body>
<script>
 function count(){
 let n1 = document.getElementById("n1").value;
 let opt = document.getElementById("opt").value;
 let n2 = document.getElementById("n2").value;
 document.getElementById("res").innerHTML = eval(n1+opt+n2)
 }
</script>
</html>